1. Field of the Invention
The present invention relates to an image forming apparatus and in particular relates to an image forming apparatus whereby color offset is prevented by correcting rotational speed variations of tandem photosensitive drums.
2. Description of the Related Art
Conventional color image forming systems include: the intermediate transfer system, in which toner images are formed, one color at a time, on a single photosensitive drum and these are successively transferred onto a transfer member to form the color image; and the tandem system, in which a plurality of photosensitive drums are arranged adjacently, toner images are formed, one color at a time, on each photosensitive drum, and are transferred onto a transfer member that is successively moved past these. Of these, greatest speed can be achieved with the tandem system, since operation can be effected with a plurality of photosensitive drums substantially at the same time in synchronized fashion.
With this tandem system, since a plurality of toner images are superimposed, it is necessary to drive the plurality of photosensitive drums without rotational speed variation and in precisely synchronized fashion. Conventionally, a method was employed wherein the plurality of photosensitive drums were brought into contact with a drive gear and the drums were rotated using a single photosensitive drum drive motor. With this method, the surface speed of the photosensitive drums is greatly affected by eccentricity or pitch variation of the drive gear. The method of eliminating the effects of eccentricity or pitch variation of the drive gear by providing photosensitive drum drive motors for each respective photosensitive drum so that the respective photosensitive drums are individually driven is therefore frequently adopted.
However, even if this method is adopted, it is difficult to completely eliminate variation of the speed of rotation of the photosensitive drums due to the effects of for example manufacturing variations of the motor for driving the photosensitive drums or mounting accuracy of the photosensitive drums themselves, or eccentricity of the drum shafts. Accordingly, the technique has been proposed of reducing speed variation of the photosensitive drums by detecting the rotational speed of the photosensitive drums and subjecting this to Fourier transformation so as to extract a variation component of arbitrary frequency and correcting the rotational speed by inverse phase data of this variation component.
The color image forming apparatus disclosed in Laid-open Japanese Patent Application No. 2002-72816, in which rotational speed variation of the rotary body is prevented, constitutes drive means that drives a rotary body in rotation. A frequency signal proportional to the rotational speed of the drive means is output by first speed detection means. A frequency signal proportional to the rotational speed of the rotary body is output by second speed detection means. Fourier transformation processing of the signal that is output by the second speed detection means is performed by Fourier transformation means. The correction data calculation means extracts a specific frequency component that is the subject of correction, and calculates and generates correction data from the frequency and amplitude thereof. The correction data calculated by the correction data calculation means is stored in correction data storage means. Drive control means controls the speed of rotation of the drive means using the detection signals of the first speed detection means and second speed detection means, and the correction data that is read from the correction data storage means.
The color image forming apparatus disclosed in Laid-open Japanese Patent Application No. H10-119355 reduces overshoot and undershoot generated when the speed of rotation of a polygonal motor is changed. An encoder detection circuit outputs encoder pulses in response to variation of the rotational speed of a photosensitive drum. A timer circuit outputs the amount of variation (count data) of the rotational speed of the photosensitive drum in unit control time (between encoder pulses). A calculation circuit finds the speed data by converting from count data to polygonal mirror rotational speed, and stores the result in RAM. An addition circuit adds reference rotational speed amount data to the speed data. A drive pulse generator generates a plurality of control signals from, the data obtained by the addition circuit. The polygonal motor driver controls the polygonal motor.
In the color image forming apparatus disclosed in Japanese Patent Number 3186610, AC color range offset can be reduced by suppressing to an appropriate and satisfactory extent cyclical rotational variations produced by for example eccentricity caused by the various types of rotary bodies themselves, such as the image carrier constituted by the photosensitive drum, transfer belt or intermediate transfer body belts, that are driven in rotation, or that are caused by the mounting thereof, eccentricity due to clearance errors of the drive shafts of the rotary bodies, or unevenness of belt thickness. The phase difference of the position of latent image writing on the image carrier and the transfer position is roughly 180°. A pattern for detection purposes that is formed on the endless carrier or the like is detected by pattern detection means. The drive control means performs control so as to effect individual fine adjustment of the rotational speeds of the rotary bodies such as the image carrier or endless carrier such as to cancel the variations of rotational speed thereof, by using amplitude component information relating to cyclical variation of rotational speed obtained by the detection means.
However, although speed variations of arbitrary period can be suppressed with these conventional methods, they cannot be completely removed. Also, two rotational speed detection devices are necessary for each photosensitive drum drive system. A further problem is that the device is made complicated by Fourier transformation of the rotational speed of the photosensitive drum, extraction of the correction frequency and computation of the correction data. A further problem is that, since control is not performed between the photosensitive drums, any speed variation component that has not been completely removed directly gives rise to color offset, which appears in the image.